The overall objective of this proposal is to gain greater understanding of how "T" cells participate in the pathogenesis or prevention of periapical lesions elicited by Streptococcus faecalis. Several experiments suggest that it is essential to understand the interplay between "T" cells and the cells found in the pulp, furcal and periapical lesions. "T" cells comprise many functional sub-populations of lymphocytes that are engaged in delayed hypersensitivity, rejection of foreign tissue grafts, tumor, elimination of bacteria and viral infected cells. They also amplify (T helper cells) or suppress "B" cell responses. Specifically, this proposal seeks support to delineate the role that "T" cells play in furcal, periapical and pulpal lesions induced in mice by S. faecalis. Established techniques of immune reconstitution along with histological procedures will be employed to determine whether "T" cells alter the course of pulp, furcal and periapical lesions in athymic mice. Advantage will be taken of the depleted "T" cell levels found in nude mice to characterize their functions.